“It’s not the heat… it’s the humidity that gets you.” Not only is that true for personal comfort, but even more so for important items that need to be kept at a specific humidity (like cigars at 70%) or metal parts on guns (which want to be stored in a dry location). I don’t smoke cigars, but I do keep firearms in a safe, and so I purchased a Caliber 4R digital hygrometer and thermometer on Amazon for $25, with the intent to mount it inside my gun safe.

Caliber 4R digital hygrometer and thermometer

$25 is a pretty good deal, and so I was somewhat skeptical as to how accurate this device could be. The box claimed “Advanced humidity sensor maintains accuracy to +/- 1% @ 70% RH” (relative humidity). To test this claim, I resorted to the tried and true “salt test.” It’s simple, accurate, and inexpensive. All you need is:

some table salt

a small plastic lid from an empty milk jug, soda bottle, or water bottle

a zip-lock bag

some distilled water

Some patience

The science behind this test is that a wet sodium chloride (table salt) mixture will establish a relative humidity in an enclosed space of precisely 75%. By creating that environment inside a plastic bag, you can check to see how far off from 75% your hygrometer is, and then calibrate it accordingly. Here’s how you do it:

Step 1: Fill the empty bottle cap about 3/4 full of salt.

Step 2: Pour a very small amount of distilled water into the cap, to create a slurry. You don’t want all the salt to dissolve. You want it to appear grainy and gooey, like wet sand at the beach just after the surf has uncovered it (see the photo below for how it should look). You can pour off excess water if you put too much in, or play with the water/salt mixture to get it right.

Step 3: Place the lid inside the zip-lock bag, then place the hygrometer inside the bag, display side up, near to the bottle cap. The technically “ideal” location for measurement is suspended 3 inches above the salt mixture, but that level of accuracy isn’t necessary for a $25 device. Placing it right next to the bottle cap is fine.

Step 4: Seal the bag, but don’t push any air out of it. Whatever normal amount of air is naturally inside the bag as you put stuff inside is fine. Place the bag somewhere that’s not too warm — ideally around 70F — that won’t fluctuate much in temperature.

Step 5: Leave the bag overnight. At a minimum, the bag should stay sealed for 6-8 hours. Ideally, 12-24 hours would be great. You want to make sure the humidity inside the bag has stabilized at 75%.

Caliber 4R hygrometer in a salt test

Step 6: Read the relative humidity level on your hygrometer. If it reads exactly 75% — you’re done! If it’s too high or too low, take it out of the bag and use whatever adjustment method is available (buttons for digital, adjustment screw for analog) to set it to exactly 75%. You now have a perfectly calibrated hygrometer!

You can actually use this method to calibrate multiple hygrometers at once (as many as you can fit in the bag). Experts suggest that you re-calibrate a hygrometer every 6 months, but for my application I’ll probably only do it every year or two.